Method and device for measuring sound wave propagation time between loudspeaker and microphone

ABSTRACT

A device  1  for measuring a propagation time of a sound wave comprises a sound source means  11  and a calculation means  12 . The sound source means  11  outputs a time stretched pulse as a sound source signal input to a speaker  3 . The calculation means  12  calculates a cross-correlation function of the time stretched pulse and the sound signal which is output from the speaker  3  and is received in a microphone  4 . Based on the cross-correlation function, the propagation time of the sound wave between the speaker  3  and the microphone  4  is found.

The present application claims the benefit of priority of InternationalPatent Application No. PCT/JP2003/015702 filed on Dec. 9, 2003, whichapplication claims priority of Japanese Patent Application No.2002-357095 filed Dec. 9, 2002. The entire text of the priorityapplication is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method and device for measuring apropagation time of a sound wave between a speaker and a microphone.

BACKGROUND ART

In some cases, it is necessary to measure a propagation time of a soundwave from a speaker to a microphone in a space in which an acousticsystem is installed. This corresponds to, for example, cases where afrequency characteristic of the acoustic system is measured at alistening position, and a signal having a frequency characteristic thatvaries with time is used as a sound source signal for measurement. Insuch cases, measurement with higher precision is sometimes achieved bytaking in a signal from the microphone installed at the listeningposition after passing the signal through a filter that varies itsfrequency characteristic according to a time variation in the frequencycharacteristic of the sound source signal for measurement, rather thanby directly taking in the signal from the microphone installed at thelistening position. In this case, it becomes necessary to delay thevariation in the frequency characteristic of the filter by time forwhich the sound wave propagates over a distance from the speaker to thelistening position, instead of simultaneously progressing the variationin the frequency characteristic of the sound source signal formeasurement and the variation in the frequency characteristic of thefilter. For this purpose, it is necessary to measure the propagationtime of the sound wave from the speaker to the microphone installed atthe listening position.

Accordingly, there has been conventionally proposed a method ofmeasuring a propagation time of a sound wave between a speaker and amicrophone using a pulse (see for example, Japanese Laid-Open PatentApplication Publication No. 2001-112100 (see page 3, FIGS. 1 and 2)).Specifically, a propagation time of a pulse sound which is output fromthe speaker and arrives at the microphone is measured.

Measurement using the pulse sound can be conducted with relativelyhigher precision unless it is affected by a noise. However, since thepulse sound has a small energy with respect to its amplitude, it isdifficult for the microphone to receive the sound with a preferred S/Nratio. In this method, therefore, accurate measurement is not alwaysconducted.

In order to improve this method, the applicant has made an attempt tomeasure a propagation time of a sound wave having a sweep signal as asound source, as a signal having a relatively large energy with respectto its amplitude. Specifically, the sweep signal which isfrequency-swept in a short time is input to a speaker, which outputs asweep sound, which is received by a microphone. And, arrival time of thesound wave is measured for each frequency band.

If the sweep signal as the sound source signal is known, it is possibleto know when a component in each frequency band is output from thespeaker. In addition, it is possible to know arrival time of thecomponent in each frequency band by band pass filtering the signalreceived by the microphone.

By finding an effective value of the signal in each frequency bandreceived by the microphone for a fixed duration while slightly shiftinga time starting point, a root-means square (RMS) value as a function ofthe time starting point may be found, and a time point at which the RMSvalue becomes maximum may be assumed to be the arrival time of thecomponent in each frequency band. This enables more accurate measurementof a distance.

This method has advantages as follows: {circle around (1)} A frequencyband with a higher level can be selected because of the use of aplurality of frequency bands. {circle around (2)} Interference from anoise is less because of the use of the band pass filter. {circle around(3)} The sweep signal is resistant to a noise because it has an energylarger than that of the pulse.

On the other hand, this method has disadvantages as described below. Theresponse is slow because of the use of the band pass filter. Ameasurement value may be corrected in view of a known delay of aresponse time. But, if the response time of the band pass filter islarger than the propagation time of the sound wave between the speakerand the microphone, measurement precision is not ensured. While thesignal is less affected by the noise as the frequency band of the bandpass filter decreases, the response time of the band pass filterincreases.

The response time of the band pass filter decreases as the frequencyband of the band pass filter increases, but the signal is susceptible tothe noise. Further, a frequency characteristic of an acoustic system inthat frequency range may appear, which may cause a peak value of thesignal in a frequency other than a target frequency to be detected. Thismay lead to inaccurate measurement.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above mentionedproblems, and an object of the present invention is to provide a methodand device for measuring a propagation time of a sound wave, which isless susceptible to a noise or a delay time of equipment and is hencecapable of accurate measurement.

In order to solve the above mentioned problems, a method of measuring apropagation time of a sound wave between a speaker and a microphone,according to the present invention, comprises: a first step ofoutputting a time stretched pulse from the speaker; a second step ofreceiving a sound signal output from the speaker in the microphone andtaking in the received sound signal from the microphone; and a thirdstep of calculating a cross-correlation function of the time stretchedpulse and the received sound signal taken in in the second step, whereinthe propagation time of the sound wave between the speaker and themicrophone is found based on the cross-correlation function. Inaddition, in order to solve the above mentioned problems, a device formeasuring a propagation time of a sound wave between a speaker and amicrophone, according to the present invention, comprises: a soundsource means; and a calculation means, wherein the sound source means isconfigured to output a time stretched pulse as a sound source signalinput to the speaker, and the calculation means is configured to takein, from the microphone, a sound signal which is output from the speakerand is received in the microphone, and to calculate a cross-correlationfunction of the time stretched pulse and the received sound signal takenin, and to find the propagation time of the sound wave between thespeaker and the microphone based on the cross-correlation function.

In accordance with such a method and device, the time stretched pulse isused as the sound source signal. The time stretched pulse is lesssusceptible to a noise because of its relatively large energy withrespect to its amplitude. Therefore, a measurement value of thepropagation time of the sound wave by the above method and device hashigh reliability. Also, it is known that the cross-correlation functionof the time stretched impulse and the response waveform to which thetime stretched pulse is input conforms to an impulse response in thatsystem. As a result, measurement is conducted with precisionsubstantially as high as that with which measurement is conducted usingthe impulse.

In the method of measuring a propagation time of a sound wave between aspeaker and a microphone may further comprise a fourth step of detectinga time when the cross-correlation function has a maximum value, a timewhen the cross-correlation function has a minimum value, or a time whenthe cross-correlation function has a maximum absolute value. In thedevice for measuring a propagation time of a sound wave between aspeaker and a microphone, the calculation means may be configured todetect a time when the cross-correlation function has a maximum value, atime when the cross-correlation function has a minimum value, or a timewhen the cross-correlation function has a maximum absolute value.

In the method of measuring a propagation time of a sound wave between aspeaker and a microphone, the first step, the second step, and the thirdstep may be performed plural times, and the method may further comprise:a fifth step of synchronizing and adding a plurality ofcross-correlation functions obtained in the third step performed pluraltimes, wherein the propagation time of the sound wave between thespeaker and the microphone may be found based on the cross-correlationfunction obtained by synchronizing and adding the plurality ofcross-correlation functions. In the device for measuring a propagationtime of a sound wave between a speaker and a microphone, the soundsource means may be configured to output the time stretched pulse pluraltimes, and the calculation means may be configured to calculate thecross-correlation function for each time stretched pulse output from thesound source means, to synchronize and add cross-correlation functions,and to find the propagation time of the sound wave between the speakerand the microphone based on the cross-correlation function obtained bysynchronization and addition.

In accordance with such a method and device, the synchronization andaddition enable measurement with high reliability.

The above and further objects and features of the invention will be morefully be apparent from the following detailed description with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing a construction of a device formeasuring a propagation time of a sound wave and an acoustic system; and

FIG. 2 is a view schematically showing a calculation content of acalculation and control portion.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described with referenceto the drawings.

FIG. 1 is a view schematically showing a construction of an embodimentof a device according to the present invention and an acoustic system tobe measured. A device (device for measuring a propagation time of asound wave between a speaker and a microphone) 1 of FIG. 1 is capable ofcarrying out an embodiment of a method of the present invention (methodof measuring a propagation time of a sound wave between a speaker and amicrophone).

The device 1 comprises a DSP (digital signal processor), an A/Dconverter, a D/A converter, and the like. In FIG. 1, the device 1 isillustrated as including a sound source portion 11 and a calculation andcontrol portion 12, giving attention to main function of the device 1.

The device 1 is configured to measure a propagation time of a sound wavebetween a speaker 3 and a microphone 4. An amplifier 2 and the speaker 3form a part of an acoustic system installed in an acoustic space (e.g.,music hall, gymnastic hall, or playing field). The microphone 4 isinstalled at a listening position (e.g., position of seat on whichaudience sits) in this acoustic space. As the microphone 4, a noisemeter may be used. The microphone 4 is located to be spaced a distance Lapart from the speaker 3. The distance L is unknown, but can becalculated if the propagation time of the sound wave between the speaker3 and the microphone 4 can be measured.

A sound source signal is output from the sound source portion 11 to theamplifier 2. The amplifier 2 power-amplifies the signal and outputs theamplified signal to the speaker 3, which radiates the signal asamplified sound. The microphone 4 receives the amplified sound outputfrom the speaker 3. The microphone 4 outputs a signal to the calculationand control portion 12.

The calculation and control portion 12 is configured to control thesound source portion 11. More specifically, the sound source portion 11receives a command signal from the calculation and control portion 12and outputs a time stretched pulse (hereinafter simply referred to as“TSP”) as a sound source signal. The TSP refers to a signal which isstretched in a time axis direction by varying a phase of an impulse inproportion to a square of a frequency.

FIG. 2 is a view schematically showing a calculation content of thecalculation and control portion 12.

The calculation and control portion 12 pre-stores a waveform of the TSPand causes the sound source portion 11 to output the TSP. In FIG. 2, thewaveform of the TSP is represented by X. The TSP is stored as 128 sampledata in the calculation and control portion 12. Sampling frequency ofthe TSP is 48 kHz, and therefore, duration of the TSP is about 2.7 msecond. The TSP has an even amplitude characteristic up to 5 kHz.

The calculation and control portion 12 outputs data of the TSP to thesound source portion 11, and outputs the command signal to the soundsource portion 11 to cause the sound source portion 11 to output theTSP. At the same time, the calculation and control portion 12 startssampling of the signal (signal indicated by Y in FIG. 2) output from themicrophone 4. Sampling frequency is 48 kHz and sampling period is 0.5second.

After an elapse of time ts after the calculation and control portion 12has output the command signal to cause the sound source portion 11 tooutput the TSP, the sound source portion 11 outputs the TSP. In otherwords, after the elapse of the time ts after the calculation and controlportion 12 has started sampling of the signal output from the microphone4, the sound source portion 11 outputs the TSP. This delay time tsoccurs due to the A/D converter and the D/A converter included in thesound source portion 11, and is recognized (stored) in the calculationand control portion 12. Hereinafter, this time ts is referred to as“sound source output delay time ts.”

The calculation and control portion 12 calculates a cross-correlationfunction of the waveform of the TSP pre-stored therein and the signalwaveform which has been output from the microphone 4 and sampled.

The following formula (formula 1) is a calculation formula of thecross-correlation function.

$\begin{matrix}{R_{(m)} = {\frac{1}{N\;\delta_{X}\delta_{Y}}\;{\sum\limits_{n = 0}^{N - 1}{X_{(n)} \cdot Y_{({n + m})}}}}} & \left( {{formula}\mspace{14mu} 1} \right)\end{matrix}$

In the above formula (formula 1), N is the number of times sampling isperformed, and δX and δY are standard deviations in X(n) and Y(n),respectively.

In FIG. 2, R represents the cross-correlation function obtained bycalculation according to the above formula (formula 1).

The calculation of the cross-correlation function may be performed afterthe signal output from the microphone 4 has been sampled for 0.5 secondand all the data corresponding to 0.5 second have been sampled, orotherwise, may be performed for each sampling using 128 sample datasampled most recently while sampling the signal output from themicrophone 4. This is because, the calculation of the cross-correlationfunction can be started when at least 128 sample data of the signaloutput from the microphone 4 has been stored, since the TSP output fromthe sound source portion 11 is 128 samples.

When the TSP is input to a system and a response waveform thereof isobtained, the cross-correlation function of the TSP and the responsewaveform thereof conforms to an impulse response of the system.Therefore, it may be assumed that the calculation and control portion 12calculates the impulse response of the system.

The cross-correlation function R may be found only for one TSP outputfrom the sound source portion 11. Nonetheless, precision improves if thecross-correlation functions R are found for respective of the TSPsoutput plural times (several times), and are synchronized and added. InFIG. 2, Ra represents a cross-correlation function obtained bysynchronizing and adding, and averaging the cross-reference functions Routput plural times.

The calculation and control portion 12 detects a time when the waveformof the cross-correlation function Ra obtained by synchronization andaddition has a maximum value. The waveform of the cross-correlationfunction Ra of FIG. 2 has the maximum value at time t1. This time t1 maybe assumed as the delay time in the whole system of FIG. 1. Hereinafter,the time t1 when the cross-correlation function has the maximum value isreferred to as “total delay time t1.”

The total delay time t1 includes the above mentioned sound source outputdelay time ts and time tb (hereinafter referred to as “spatial delaytime tb”) for which the sound wave propagates through a space rangingfrom the speaker 3 to the microphone 4. It shall be appreciated that adelay time elapsed from when the signal is input to the amplifier 2until when the signal vibrates a diaphragm of the speaker 3 or a delaytime elapsed from when a diaphragm of the microphone 4 starts vibratinguntil when the signal caused by the vibration appears at an outputterminal of the microphone 4 is negligible small in contrast to thespatial delay time tb. When the spatial delay time tb is measured foradjustment or measurement of the acoustic system including the amplifier2 and the speaker 3, it is convenient to include, in the spatial delaytime tb, the delay time elapsed from when the signal is input to theamplifier 2 until when the signal vibrates the diaphragm of the speaker3.

As described above, since the calculation and control portion 12pre-stores the sound source output delay time ts, the spatial delay timetb can be calculated by detecting the total delay time t1. According toa procedure shown in FIG. 2, synchronization and addition are performedto obtain the cross-correlation function Ra, the time t1 when thecross-correlation function Ra has the maximum value is detected, and thespatial delay time tb is obtained by subtracting the sound source outputdelay time ts from the total delay time t1. This is represented by aformula: “tb=t1−ts.” The spatial delay time tb is multiplied by a soundspeed c to obtain a distance between a point where the speaker 3 isinstalled and a point where the microphone 4 is installed.

If the sound source output delay time ts is negligible small in contrastto the spatial delay time tb, then the total delay time t1 may beassumed to be the spatial delay time tb. If the calculation and controlportion 12 starts sampling the signal output from the microphone 4 atthe same time the sound source portion 11 starts outputting the TSP, thesound source delay time ts may be assumed to be 0.

As described previously, when the TSP is input to a system and aresponse waveform thereof is obtained, the cross-correlation function ofthe TSP and the response waveform thereof conforms to the impulseresponse in that system, and therefore, it may be assumed that thecalculation and control portion 12 calculates the impulse response inthat system. Therefore, the device 1 for measuring the propagation timeof the sound wave shown in FIG. 1 is capable of measuring thepropagation time of the sound wave between the speaker 3 and themicrophone 4 with precision substantially as high as that with whichmeasurement is conducted using the impulse. In addition, since theenergy of the sound source signal is less susceptible to the noisebecause of its relatively large energy, the propagation time of thesound wave between the speaker 3 and the microphone 4 can be measuredwith high precision.

Thus far, one embodiment of the present invention has been described.While in the above embodiment, the cross-correlation function iscalculated according to the formula (1), it may alternatively becalculated according to a formula (2) in which a calculation portion((1/N ·δX·δY) portion) for normalization in the formula (1) is omitted.

$\begin{matrix}{R_{(m)} = {\sum\limits_{n = 0}^{N - 1}{X_{(n)} \cdot Y_{({n + m})}}}} & \left( {{formula}\mspace{14mu} 2} \right)\end{matrix}$

While in the above embodiment, the time when the cross-correlationfunction obtained by synchronization and addition (or by averaging ofcross-correlation functions) has the maximum value is detected as thetotal delay time, a time when a cross-correlation function found foronly one TSP output from the sound source portion 11 has the maximumvalue may alternatively be detected as the total delay time, withoutsynchronization and addition.

While in the above embodiment, the time when the cross-correlationfunction has the maximum value is detected to find the time when thepeak appears on a plus side of the cross-correlation function and isassumed as the total delay time, a time when the cross-correlationfunction has a minimum value may be detected to find a time when thepeak appears on a minus side and may be assumed as the total delay time.Further, a time when the cross-correlation function has a maximumabsolute value may be detected and may be assumed as the total delaytime.

Numerous modifications and alternative embodiments of the invention willbe apparent to those skilled in the art in view of the foregoingdescription. Accordingly, the description is to be construed asillustrative only, and is provided for the purpose of teaching thoseskilled in the art the best mode of carrying out the invention. Thedetails of the structure and/or function may be varied substantiallywithout departing from the spirit of the invention and all modificationswhich come within the scope of the appended claims are reserved.

INDUSTRIAL APPLICABILITY

A method and device for measuring a propagation time of a sound wavebetween a speaker and a microphone of the present invention areadvantageous in technical fields of acoustic systems, since thepropagation time of the sound wave between the speaker and themicrophone can be accurately measured.

1. A method of measuring a propagation time of a sound wave between aspeaker and a microphone, comprising: a first step of outputting a timestretched pulse from the speaker; a second step of receiving a soundsignal output from the speaker in the microphone and taking in thereceived sound signal from the microphone; and a third step ofcalculating a cross-correlation function of the time stretched pulse andthe received sound signal taken in the second step, wherein thepropagation time of the sound wave between the speaker and themicrophone is found based on the cross-correlation function.
 2. Themethod of measuring a propagation time of a sound wave between a speakerand a microphone according to claim 1, further comprising: a fourth stepof detecting a time when the cross-correlation function has a maximumvalue, a time when the cross-correlation function has a minimum value,or a time when the cross-correlation function has a maximum absolutevalue.
 3. The method of measuring a propagation time of a sound wavebetween a speaker and a microphone according to claim 1, wherein thefirst step, the second step, and the third step are performed pluraltimes, the method further comprising: a fifth step of synchronizing andadding a plurality of cross-correlation functions obtained in the thirdstep performed plural times, wherein the propagation time of the soundwave between the speaker and the microphone is found based on thecross-correlation function obtained by synchronizing and adding theplurality of cross-correlation functions.
 4. A device for measuring apropagation time of a sound wave between a speaker and a microphone,comprising: a sound source means; and a calculation means, wherein thesound source means is configured to output a time stretched pulse as asound source signal input to the speaker, and the calculation means isconfigured to take in, from the microphone, a sound signal which isoutput from the speaker and is received in the microphone, and tocalculate a cross-correlation function of the time stretched pulse andthe received sound signal taken in, and to find the propagation time ofthe sound wave between the speaker and the microphone, based on thecross-correlation function.
 5. The device for measuring a propagationtime of a sound wave between a speaker and a microphone, according toclaim 4, wherein the calculation means is configured to detect a timewhen the cross-correlation function has a maximum value, a time when thecross-correlation function has a minimum value, or a time when thecross-correlation function has a maximum absolute value.
 6. The devicefor measuring a propagation time of a sound wave between a speaker and amicrophone, according to claim 4, wherein the sound source means isconfigured to output the time stretched pulse plural times, and thecalculation means is configured to calculate the cross-correlationfunction for each time stretched pulse output from the sound sourcemeans, to synchronize and add cross-correlation functions, and to findthe propagation time of the sound wave between the speaker and themicrophone based on the cross-correlation function obtained bysynchronization and addition.
 7. The method of measuring a propagationtime of a sound wave between a speaker and a microphone according toclaim 2, wherein the first step, the second step, and the third step areperformed plural times, the method further comprising: a fifth step ofsynchronizing and adding a plurality of cross-correlation functionsobtained in the third step performed plural times, wherein thepropagation time of the sound wave between the speaker and themicrophone is found based on the cross-correlation function obtained bysynchronizing and adding the plurality of cross-correlation functions.8. The device for measuring a propagation time of a sound wave between aspeaker and a microphone, according to claim 5, wherein the sound sourcemeans is configured to output the time stretched pulse plural times, andthe calculation means is configured to calculate the cross-correlationfunction for each time stretched pulse output from the sound sourcemeans, to synchronize and add cross-correlation functions, and to findthe propagation time of the sound wave between the speaker and themicrophone based on the cross-correlation function obtained bysynchronization and addition.